There are a number of network applications requiring accurate frequency and/or time synchronization references in order to operate properly, for example mobile communication technologies such as Global System for Mobile Communications (GSM), Wideband Code Division Multiple Access (WCDMA) and Long Term Evolution (LTE).
The traditional timing solution is for the network nodes to synchronize their local clocks from the synchronous stream of data used to connect the network nodes together, as for instance in case of Time Division Multiplexing (TDM) based networks, but the migration of networks from TDM to cheaper packet based technologies (such as Ethernet) requires a different approach. In such a packet based network, the data from a first end node to a second end node (e.g. from a caller's base station to a call receiver's base station) typically follows a path comprising a number of hops between a set of intermediate nodes.
One solution for a packet based topology is to use a packet based method, where the timing information is carried across a packet network by sending packets containing timestamp information, i.e. timing packets. The timestamps are generated by a master (server) that has access to an accurate reference, such as a Global Positioning System (GPS) based clock.
FIG. 1 shows an exemplary timing packet-based method of distributing synchronization, where a time server 110 sends timing packets 115 out across the packet network 120, interspersed between data packets 125, to a receiving node 130, which recovers the timing information 135 from the timing packets, for use in adjusting the local clock, and the like.
Each receiving node 130 can run an algorithm that recovers the timing information 135 based on adaptive clock recovery methods, e.g. by comparing the local timing with the arrival and/or inter-arrival times of the packets (for example, as described in the standard ITU-T G.8261). The accuracy of the recovered timing information 135 is therefore affected by variable delays in the packet network 120, and one of the key requirements of the timing information recovery algorithm is to filter out the packet delay variation (PDV).
When time synchronization is requested, a two-way timing protocol is mandatory, for example Network Timing Protocol (NTP) or Precision Timing Protocol (PTP), in which the transfer delay from master to slave is calculated.
One possible approach to network synchronisation in packet networks is to add timing support in the network nodes between the Master and the Slave. In the case of PTP, these functions are the Boundary Clocks (BC) and Transparent Clocks (TC), as described in the Institute of Electrical & Electronics Engineers standard, IEEE 1588.
FIG. 2 shows a packet-based mobile communications network including such timing support. The timing support concerns adding hardware, as well as software, functions in the intermediate network nodes (for example, Ethernet switches or routers). For example, a PTP grandmaster clock node 110 provides timing packets 115 to a base station 210, via a number of hops between network nodes (e.g. via boundary clock 111) that can process the timing packets so as to improve the overall performance of the packet based synchronisation method.
In this example the transparent clock provides a means of measuring the delay that has been added by the respective network node, and of measuring the delays on links connected to that network node. The end-equipment can use this information to recover the time reference. The boundary clock, by contrast, terminates and regenerates timestamp packets, so that internal queuing delays are reduced.
However, there are issues arising from these two new functions, Boundary Clocks and Transparent, which include:
The new functions generally require significant architectural changes.
The implementation of the Transparent Clock type of function is especially considered critical mainly due to the fact it creates a network layer violation.
The Transparent Clock in particular can also lead to security issues, as packets are modified even if not terminated in the network node, which typically means that the use of Transparent Clocks in Telecoms applications may not be accepted.
The use of Boundary Clocks in a multi-operator environment, for example where two mobile networks share radio network infrastructure, is also problematic.
Accordingly, it would be desirable to provide and improved network node apparatus and methods which reduce the packet delay variation and the impact of asymmetry in packet based networks.
Another fundamental aspect is the generation of accurate time stamps. Traditionally, the accurate generation of time stamps is achieved either via a two-step clock approach (e.g. PTP Follow-up message) or via hardware time-stamping (i.e. the timestamp is generated by the network node at the physical layer just before the packet leaves the master, and not by software implemented in networking layers higher up the networking protocol stack).
It would also be desirable to provide accurate, yet simplified approach to generation of the time stamps from a Master clock network node.